The invention relates to an image receiving material for dye diffusion thermal transfer, as well as a process for its manufacture.
A system of dye diffusion thermal transfer ("D2T2") has been developed in recent years which makes possible the reproduction of an electronically created picture in the form of a "hardcopy". The principle of D2T2 is that, with regard to the basic colors cyan, magenta red, yellow and black, the digital picture is encoded into electrical signals which are then transmitted to a thermal printer and translated into heat. The dye of the donor layer of a dye transfer band/sheet which is in contact with the receiving material sublimates under the effect of heat and diffuses into the receiving layer.
As a rule, a receiving material for dye diffusion thermal transfer comprises a support material with a receiving layer applied to its front side. Additionally other layers may be applied onto the front side, such as barrier, release, adhesive and protective layers. The necessity of such additional coatings is required by the demands placed upon the receiving material. These may be:
a smooth surface PA0 heat and pressure stability PA0 light stability (no yellowing) PA0 good dye solvency PA0 good anti-scratch and abrasion characteristics PA0 "anti-blocking" characteristics (no sticking)
Either plastic foils such as polyester film or coated paper may serve as the support material.
The main component of the receiving layer is, as a rule, thermoplastic resin showing an affinity to the dye contained in the dye transfer band. Materials suitable for such may be linear polyesters, e.g. polyethylene terephthalate, polybutylene terephthalate or acrylic resins, e.g. polymethylmethacrylate, polybutylmethacrylate, polymethylacrylate etc. Furthermore, such materials as polystyrene, polycarbonate, polyvinyl pyrrolidone, ethyl cellulose, polysulfone and other polymers may be used as dye receiving resins.
U.S. Pat. Nos. 4,748,150 and 4,774,224 show that polycarbonate may be used as a receiving layer on a polyethylene coated base paper. Moreover, an intermediate layer is applied between the support material and the receiving layer. This intermediate layer is a vinylidene chloride copolymer and serves to improve adhesion between the receiving layer and the support material. The above mentioned receiving sheet has shown itself to be disadvantageous as the polycarbonate used shows a strong tendency to yellowing and in time affects the transferred picture negatively. A further disadvantage is that both coats must be applied using solvent agents which can lead to health and safety problems.
The problem of pressure sensitivity of the receiving sheet when in contact with the heating head has been dealt with in the European Patent Application EP 0 288 193. This pressure sensitivity makes itself shown in a reduction of the surface gloss of the layer or in the phenomenon "strike-through" in which an impression of the picture can be seen on the reverse side of the receiving sheet. The problem is solved by applying a release layer based on silicone with a SiO.sub.2 additive onto a polyester receiving layer which has been coextruded onto a polyester support material. The disadvantage of the above mentioned is that the picture is blurred, probably as a result of a reaction between the reacting groups of the silicone compounds and the diffused dyes in the receiving layer. Furthermore, the similarity to a photo, as required by the market, is missing with pictures produced in this manner.
It is furthermore a fact that dye issuing from the dye donor band and diffused into the receiving layer tends to pale under the influence of light. This problem has been dealt with in the U.S. Pat. No. 4,775,657 in so much as the receiving layer, consisting of polycarbonate, is coated with a protective coating of polyester or polyurethane. A disadvantage of material so produced is the pressure sensitivity of the receiving sheet, as well as the necessity of several work operations and the necessity of using organic solvents during coating.
The Patent Application EP 0 261 970 describes a receiving layer containing a linear saturated polyester as a binding agent and a silica coupled silane copolymer as a release agent (anti-blocking additive).
The object of this invention, therefore, is to provide a receiving material for dye diffusion thermal transfer procedures which does not show the disadvantages as mentioned above, i.e. it must exhibit good heat and light proof characteristics, as well as being impervious to pressure and demonstrates good flatness and antiblocking characteristics. Moreover, the receiving material represents a further improvement of color density and color gradation compared to the receiving sheets already on the market.
The object of the invention is accomplished by coating the front side of polyolefin coated base paper with a receiving coating mass which, as a dye receiving resin, contains a combination of at least one acrylate copolymer polar group and oxidized polyethylene.
It was surprising to find that the utilization of the above mentioned combination created a receiving material which not only met the requirements as previously listed, but at the same time enabled a high color density of the printed image, as well as improving color gradation.
In a preferred embodiment of the invention an acrylate copolymer whose polar groups are carboxyl, metal combined carboxyl groups and/or nitrile groups was utilized. Zinc combined carboxyl groups are especially to be preferred in the metal combined groups.
Acrylonitrile and/or methacrylic acid take part in the structure of the acrylate copolymer used in the combination according to the invention and the amounts of these monomers in the copolymers are between 10 and 40 mol %. In a preferred embodiment the amounts of these monomers are between 25 and 35 mol %. The acrylate copolymer compound may, additionally, contain styrene in an amount of up to 40 mol %.
The weight relationship of the acrylate copolymer to the oxidized polyethylene in the combination according to the invention may be between 99:I and 30:70. The best results, as far as color density and color gradation are concerned, were obtained with acrylate copolymer/oxidized polyethylene in a weight relationship of between 70:30 and 40:60 (see Example 2, Table 2).
The receiving layer for the receiving sheet according to the invention may contain, as well as the dye receiving resin, fine particled silica or Al.sub.2 O.sub.3 as a matting agent or further additives, such as fluorine tenside as wetting agents, dispersing agents, dye couplers, UV stabilizers, pigments and other auxilary agents.
The coating mass for the receiving layer may be applied using any of the usual procedures for coating and dosing such as roll gravure, nipcoating, air brushing or wire bar onto a substrate, as for instance polyethylene coated paper. The receiving coating may be applied in an aqueous form in a one step operation. The coating weight of the receiving layer may be between 0.3-15 g/m.sup.2, but 1-10 g/m.sup.2 is preferable.
As a support material, a paper with at least one side coated with a polyolefin, such as polyethylene, is preferred, wherein this polyolefin layer applied in accordance to the available coating technology has a basis weight of more than 5 g/m.sup.2, and preferably between 7-25 g/m.sup.2.
The polyolefin layer may contain pigments and other additives.
The invention is illustrated in the following examples, although this in no way sets limits to the invention.